The two antitumor agents, Cyclophosphamide and Isophosphamide, are chiral organophosphorus compounds which require microsomal activation in vivo to their cytotoxic forms. The high selectivity of these cytotoxic metabolites to neoplastic tissue has been credited to the presence or absence of additional metabolic processes which form non-toxic substances. We are proposing to synthesize cyclophosphamide, isophosphamide, and their chiral metabolites in optically active forms to test whether enzyme stereospecificity in the metabolic processes has an important effect on the action of these clinically useful agents.